1. Field of the Invention
The present invention relates to a semiconductor laser element having an Inx3Ga1-x3As1-y3Py3 active layer which is formed above a GaAs substrate and lattice-matches with GaAs.
2. Description of the Related Art
Recently, high-power semiconductor laser elements are used in the fields of image processing, printing, and medicine. Semiconductor laser elements used in these fields are required to operate with output power from 100 mW to 1 W or higher and high reliability, and there are demands for increasing the output power of the semiconductor laser elements.
Generally, when a semiconductor laser element operates with high output power, the so-called catastrophic optical mirror damage (COMD) occurs, i.e., the end facets are degraded and damaged by high optical density. In the conventional AlGaAs-based semiconductor lasers, non-radiative recombination is likely to occur due to the inclusion of Al. Therefore, currents generated by the non-radiative recombination generate heat, and the temperature at the end facet rises. Since the temperature rise at the end facet decreases the energy gap, the number of non-radiative recombination centers increases. Thus, the end facets are damaged through the above vicious cycle. This phenomenon is a factor of impeding realization of reliable, high output power operation of the semiconductor lasers.
On the other hand, Electronics Letters, vol. 34, (1998) pp.1100 discloses a InGaP-InGaAsP-based semiconductor laser which does not contain Al. Due to the absence of Al in crystals, the non-radiative recombination centers are less likely to be produced, and therefore the COMD level is high. However, the electrical resistance at the GaAs/InGaP hetero-interface is great. Therefore, the characteristics of the disclosed semiconductor laser element are not satisfactory, and the reliability is low.
In order to solve the above problems, Japanese Unexamined Patent Publication (JUPP) No. 6(1994)-302910 discloses an Al-free semiconductor laser element in which electrical resistance is reduced by using a graded-index type light-carrier-separate-confinement structure and unsymmetrically formed optical waveguide layers. However, due to the miscibility gap, it is impossible to produce crystals having satisfactory quality in the manufacturing process of the semiconductor laser element disclosed by JUPP No. 6(1994)-302910. Therefore, the electrical resistance of the entire semiconductor laser element cannot be effectively reduced. In addition, the electrical resistances at the interface between a GaAs substrate and a cladding layer and the interface between a contact layer and another cladding layer remain great.